A grinding wheel disclosed in Japanese Patent Laid-Open Publication No. H6-198564 is provided with a balancer mechanism to compensate for unbalance in weight distribution around its rotational axis. More specifically, this grinding wheel has a storage chamber and weights which are stored in the storage chamber. The inner surface of the storage chamber has a guide face which extends along the circumferential direction of rotation and wall faces disposed in one and the other of the axial directions. Each weight has a pressing face which is pressed against the guide face by means of centrifugal force generated by rotation of the grinding wheel. When the grinding wheel rotates, the weights which are pressed against the guide face via the pressing face change their positions relative to the grinding wheel in the circumferential direction of rotation, and thereby the weight are disposed in positions in which they suppress whirling motion of the grinding wheel. It is also disclosed to store liquid in the grinding wheel instead of the weights. This liquid is caused to flow when the guiding wheel rotates such that the weight distribution of the liquid is caused to vary around the rotational axis, and thus the whirling motion of the grinding wheel can be suppressed.
A rotation drive mechanism disclosed in Japanese Patent Laid-Open Publication No.2000-102211 has a weight holder integrated with the rotary body and spherical weights which are held by the weight holder in order to compensate for unbalance in the weight distribution around the rotational axis of the rotary body. A guide face having a cross section in a V-shaped groove and extending along the circumferential direction of rotation is provided in the weight holder. When the rotary body rotates, the spherical weights which are pressed against the guide face having a cross section in a V-shaped groove by centrifugal force change their positions relative to the rotary body in the circumferential direction of rotation, and thereby the weights are disposed in positions in which they suppress whirling motion of the rotary body.
When an impact acts upon the aforementioned conventional grinding wheel having the balancer mechanism, a problem arises in that the weights are fluctuated in the axial direction of rotation so that the grinding wheel vibrates, thereby leading to a reduction in grinding precision. Further, if prevention of the fluctuations of the weights in the axial direction is intended while permitting the change of their positions in the circumferential direction by means of the wall faces disposed in one and the other of the axial directions of the weight, then the wall faces should be manufactured with an extremely high degree of precision and manufacturing costs rise. Even if the wall faces are manufactured with a high degree of precision, it is impossible to completely eliminate fluctuations of the weights in the axial direction of rotation due to tolerance, and thus improvement in the grinding precision is inhibited.
When an impact acts upon the aforementioned conventional rotary body integrated with the weight holder which is provided with the guide face having a cross section in a V-shaped groove, a problem arises in that since the weights are spherical, the weights are pressed evenly against one side and the other side of the guide face in the axial direction, thus causing the rotary body to vibrate due to fluctuations of the weight.
Therefore, in a case where a work is machined by a cutting tool which is attached to the spindle of a machine tool via a tool chuck, even if the whirling motion of the tool chuck is suppressed by the conventional balancer mechanism, the amplitude thereof increases when an impact due to a reactive force, which is caused by the cutting or an external factor such as vibration of vehicles travelling nearby roads, acts upon the tool chuck via the cutting tool; with the result that improvements in the working precision are inhibited and noise is generated from the bearing of the spindle, leading to a decrease in the longevity of the bearing.
When liquid is used in place of such weights, vibration in the grinding wheel caused by the impact can be attenuated by the flow of the liquid, but liquid with a large specific gravity is expensive and difficult to handle, causing an increase in costs. If a liquid with a small specific gravity such as water is used, the balancer should be increased in size in order to achieve a desired performance.
An object of the present invention is to provide a balancer which is capable of solving the above-mentioned problems.